The invention relates to a coding unit for determining how the amplitude and phase of two carrier signals should be modulated so as to transmit a data symbol by means of those carrier signals.
FIG. 1 illustrates schematically the channel encoding system in one example of a dual carrier modulation (DCM) transmitter. A bit stream 1 for transmission is passed to an encoding unit 2 which generates two streams 3, 4 of QPSK (quaternary phase shift keyed) data. Those streams pass to a decision matrix 5 which generates two streams 6, 7 of 16QAM (16 state quadrature amplitude modulation) data. The matrix 5 operates in such a way that both QAM streams are dependent on both QPSK streams, with the result that if either one of the QAM streams is received perfectly then a receiver can recover both QPSK streams from it. Each QAM stream is used to modulate a respective tone 8, 9 and the resulting signals are combined and transmitted from an antenna 10.
This scheme is used in the modern UWB (ultra-wideband) protocol. FIG. 2 illustrates a channel of that system. The channels are 528 Mhz wide. When a particular channel (e.g. channel 20) is being used the tones 21, 22 are spaced 210 MHz apart from each other.
One reason for transforming the QPSK data streams to QAM data streams in this way is to introduce additional diversity into the system. It would be possible to modulate two carriers directly with respective ones of the QPSK data streams. However, that would require the receiver to receive successfully at both carrier frequencies in order to fully recover the original data. In practice (as illustrated by noise level 23 in FIG. 2) it is less likely that there will be poor propagation conditions at both of the carrier frequencies than at one. The scheme described above has the advantage that it allows the original data to be recovered even if only one carrier is received.
FIG. 3 shows the improvement in bit error rate (BER) and packet error rate (PER) that may be achieved by using dual carrier modulation. The improvement seen using dual carrier modulation is due to increased diversity, which leads to a faster improvement in performance as the signal-to-noise ratio (SNR) increases.
DCM using QPSK data can be considered as a unitary transform of the original data into a four dimensional space. An example of this process is illustrated in FIG. 4. Each of the QPSK constellations is rotated by atan(½) (approximately 27 degrees). One constellation is rotated to the right and the other is rotated to the left. A constellation representing the amplitude and phase modulations corresponding to each four-bit symbol for each of the two carrier signals is then generating by taking the real part of the first QPSK constellation and the imaginary part of the second QPSK constellation for the first carrier signal and the real part of the second constellation with the imaginary part of the second constellation for the second carrier signal. This gives a 16 QAM constellation for each carrier that maps every four-bit data symbol onto an amplitude and phase modulation for transmitting that data symbol.
A carrier signal modulated according to the constellations shown in FIG. 4 thus carries all of the information needed to recover the two original QPSK data streams. In addition, the separation between constellation points in the two 16 QAM constellations is maximised, which increases the likelihood of a modulated carrier signal being correctly decoded. Finally, since the transform applied to the QPSK constellations is a unitary transform, the same power is transmitted and the Euclidean distances between transmitted words remain unchanged, so that the performance achieved by both carriers over an additive white Gaussian noise channel (AWGN) is the same.
To increase the data rate further higher order constellations than a QPSK constellation are required as inputs into a dual carrier modulation scheme. One option is to use a 16 QAM constellation as an input for the dual carrier modulation scheme. This scheme follows a similar pattern to that described above and illustrated in FIG. 4, except that it uses two rotated 16 QAM constellations as inputs. The resulting constellations that define the mapping of every eight-bit data symbol onto a signal carrier and contain 256 points. However, a constellation of 256 points has a very high dynamic, so that more bits are needed for the analogue-to-digital converter, and it is complex to decode the modulated signal because it is necessary to distinguish between the 256 points in the constellation.
Therefore, there is a need for a coding unit capable of implementing an improved modulation scheme that still offers a high data rate and the advantages of diversity but does not involve the complexity of existing schemes.